Erratic Tach readings/bad Voltage regulator?

Benny67

Chief Petty Officer
Joined
Apr 21, 2015
Messages
571
I've noticed when I am running distances (few or more miles) at cruising speed one of my tachometer is not reading the correct rpms, meaning, at around 3600 rpm (gauging this by my other motor and the similarities in sound) my gauge in question will read +/- 5000 rpm or so and has erratic movements but will sometimes go back to normal for a brief period of time. On short runs (hour or less running time) I do not see this problem nor do I see this under 3000 rpms.

my motors are twin 180 hp 3/7 liter mercuisers One has the retro fit alternator and one has the old stator/voltage regulator setup. Both have pertronix ignition setups

The one with the stator is the one giving me these issues.

The same engine also seemed to studder slightly when I am seeing this issue.

I'm assuming it's the voltage regulator going bad and when it gets hot it starts to fail but then recovers when the heat of the labouring motor backs off when I back off the engine or somehow it's drawing a heavier current when the motor sees these conditions.

I'm an Industrial electronics guy by trade so i'm not a babe in the woods in this situation.. I wanted to see what others think or have had seen in the past or if they have had this issue and to see what their solution was.

anybody?
 

bruceb58

Supreme Mariner
Joined
Mar 5, 2006
Messages
30,454
The same engine also seemed to studder slightly when I am seeing this issue.
Bad gauge. Swap the grey signal wires going to the gauges and see if the problem moves or the same gauge reads off. A bad gauge can cause ignition issues.
 

MH Hawker

Vice Admiral
Joined
Jul 13, 2011
Messages
5,516
it is often a gummed connection in the gage pull the gage out and rotate the pole or pulse switch that will clean the internal connections and it may fix it note what its set to first though
 

Benny67

Chief Petty Officer
Joined
Apr 21, 2015
Messages
571
Bad gauge. Swap the grey signal wires going to the gauges and see if the problem moves or the same gauge reads off. A bad gauge can cause ignition issues.

I don't think the gauge is bad cause both were changed 2 years ago...although that really means nothing and they COULD be bad...but it's unlikely

I actually scored a used alternator and mounting setup on ebay for cheap...ill test the alternator and if it's good, use it and if not at least I got the mounting brackets for a lot less than I would have buying them from from a supplier. I'll upgrade to the alternator and see if the issue continues. It needed to be done anyway. That stator/regulator was bound to fail at some point.

Funny thing is everyone flames that setup but mine has been working since the boat was new (1987) so it can't be THAT bad..

I also WAS running on both batteries (switch on "both") so it's possible there was some bucking issues with both charging systems and it was causing this issue.
 

Benny67

Chief Petty Officer
Joined
Apr 21, 2015
Messages
571
Well, 12:30 last night...Luckily when I was docking the motor in question died.

I went to give it a quick Rev out of gear just to hear if it studdered or reved smooth before I shut down and it just s**t the bed and wouldn't start again.

I haven't had a chance to investigate the problem yet but I'm assuming its connected to this tach issue.

I'm thinking the pertronix module died.

I'm going down tomorrow morning to sort out the situation. I'll update shortly
 

Benny67

Chief Petty Officer
Joined
Apr 21, 2015
Messages
571
Update,

Turns out the Pertronix module was flaky..I swapped both module and coil with a new one and everything works great again.

Got Amazon to send me a new module and coil under warranty and even with Sunday delivery to boot.

Installed both the new coil and module in 15 min and Im running smooth as silk again
 
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